Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of carbon monoxide and hydrogen (i.e., syn or synthesis gas) with carbon compounds containing olefinic unsaturation. The reaction is performed in the presence of a carbonylation catalyst and results in the formation of a compound, for example an aldehyde, which has one more carbon atom in its molecular structure than the starting olefinic feedstock.
By way of example, higher alcohols may be produced in the so-called "oxo" process by hydroformylation of commercial C.sub.4 to C.sub.16 olefin fractions to an aldehyde-containing oxonation product, which on hydrogenation yields respective C.sub.5 to C.sub.17 saturated alcohols. The olefin feedstocks react to form aldehydes, alcohols, formate esters and some higher boiling condensation, esterification, and dehydration by-products. Some of the olefin feedstock is also hydrogenated to form paraffins.
The catalyst normally employed is a homogeneous cobalt catalyst, hydro cobalt tetracarbonyl, i.e., HCo(CO).sub.4. The hydroformylation reactors operate typically at 120.degree.-180.degree. C. and at a pressure of 202.6-303.9 bar. At these conditions the cobalt is almost entirely in the form of hydro cobalt tetracarbonyl.
Prior to sending the hydroformylation product to the next processing step which is normally hydrogenation where the aldehydes are converted to the corresponding alcohols the cobalt catalyst must be removed. One such conventional method for removing cobalt values from a crude product is by a technique commonly referred to as "Cobalt Flash." U.S. Pat. No. 4,625,067 (Hanin), which issued on Nov. 25, 1986, discloses the Cobalt Flash process wherein the crude product is contacted with a stream of stripping gas to entrain volatile cobalt compounds. The contacting is performed in the presence of water and aqueous acid to dissolve those cobalt values not entrained in the gas under the conditions of temperature and pressure employed for the contacting, and the aqueous phase is subsequently separated from the organic hydroformylation reaction product.
Although the stripping method disclosed in the Hanin patent overcomes the disposal and chemical additive costs of the caustic/acidification method disclosed in U.S. Pat. No. 3,725,534 (Reisch), which issued on Apr. 3, 1973, and U.S. Pat. No. 5,091,599 (DeMunck et al.), which issued on Feb. 25, 1992, it has the disadvantage that when lower carbon number olefins (e.g., C.sub.7 and below) are used as feedstock, unreacted compounds such as olefins and/or paraffins are stripped out together with the volatile cobalt compounds. These olefins and/or paraffins are then absorbed into the olefinic feedstock and recycled to the oxo reactor. This occurs because lower carbon number feedstocks such as heptene have roughly the same volatility as the cobalt specie, thereby causing it to be entrained together with the volatile cobalt and taken out overhead. At atmospheric pressure the boiling point of hydro cobalt carbonyl is estimated to be 47.degree. C. which falls between the boiling points of n-pentane, i.e., 36.degree. C., and n-hexane, i.e., 69.degree. C. As a result when the hydro cobalt tetracarbonyl is stripped at 95.degree. C. and less than 10 atmospheres, as described in U.S. Pat. No. 4,625,067, the light hydrocarbons present are also taken overhead in significant quantities.
In the absorption step of Hanin, the oxo feed olefin is used as the absorbent and has the same carbon number as the stripped paraffins and olefins. As such the stripped paraffins and olefins are absorbed into the feed olefin along with the hydro cobalt tetracarbonyl, i.e., volatile cobalt carbonyl. As the oxo cycle is repeated the light hydrocarbon level in the oxonation feed rapidly increases, thus sharply decreasing the olefin content of the feed and reducing net olefin feed rates.
The present inventor has developed a novel method of recovering cobalt values by means of the Cobalt Flash mode which does not cause the build up of unreacted light hydrocarbons within the system, thereby avoiding a decrease in the net olefin feed rate. This is accomplished by replacing the olefinic feed in the absorber with a heavier molecular weight higher olefin to serve as the absorbent. Use of the heavy olefin absorbent accomplishes two primary objectives. First, the total amount of paraffin which can be absorbed is smaller. Second, the higher gas to olefin ratio will heat up the olefins as they are countercurrently contacted also decreasing the total amount of paraffin which will be absorbed, and accelerating the complexation of the cobalt with heavier olefin as the temperature increases. Therefore, a majority of the paraffins and other light hydrocarbons remain in the gas stream and exit the absorber.
Although U.S. Pat. No. 5,091,599 (DeMunc al.) is not directed to a Cobalt Flash method, it does incorporate in its caustic/acidification method a step wherein the volatile cobalt from a stripper reactor is contacted with an absorbent other than the olefinic feed to recycle the cobalt catalyst back to oxonation of C.sub.3 to C.sub.6 olefins. Traditionally the absorbent used in caustic/acidification method is the olefinic feedstock. DeMunck et al. replaces the olefinic feedstock with the residue obtained during hydroformylation. DeMunck et al. discovered that the so called U.HOF, the product left after upgrading of the heavy oxo by-products is a very effective hydro cobalt carbonyl absorption fluid, almost comparable to olefin feed and much better than the heavy oxo by-products themselves. U.HOF is the heavy product derived after subjecting the heavy oxo by-products to cracking which may be achieved by subjecting the by-product to a temperature in the range 300.degree.-350.degree. C., at low pressure and in the presence of steam and a catalyst, such as alumina.
The primary differences between the present invention and the method disclosed in the DeMunck et al. patent are: (1) DeMunck et al. include the additional steps of converting the crude oxo product into a water soluble sodium cobalt carbonyl at high temperature and high pressure by thoroughly mixing the crude oxo product with a dilute caustic solution (i.e., the caustic step), and separating the cooled and depressurized sodium cobalt carbonyl from the organic oxo products; (2) DeMunck et al. also feeds a sodium cobalt carbonyl water stream which is acidified with H.sub.2 SO.sub.4 (i.e., an organic-free water stream) to the stripper reactor whereas the present invention feeds the crude oxo product directly to the stripper reactor; and (3) the process according to DeMunck et al. involves the stripping of HCo(CO).sub.4 from a water stream only such that paraffin build-up is not a major problem, whereas paraffin build-up is a major problem in Cobalt Flash processes.
The present invention also provides many additional advantages which shall become apparent as described below.